The present invention relates to a process of clarifying water containing particles suspended therein.
It is known that, to efficiently settle particles suspended in water, reagents are introduced into the water to coagulate the particles. This coagulation is necessary to flocculate the particles and settle them to the bottom.
The use of iron salts as coagulating reagents, ferric chloride in particular, is well known. A commercially available aqueous solution of ferric chloride (FeCl.sub.3.6H.sub.2 O) in a 40 to 41% concentration by weight is generally used.
The ability of ferric chloride to destabilize colloidal suspensions, and thus its coagulating ability, is a factor of the hydrolysis that follows its dissolution, and of the intermediate formation of compounds likely to polymerize and thus activate the flocculation process. Many authors have studied this hydrolysis. Lamb et al. considered the existence of an unstable equilibrium state between the polymerized phase of ferric hydroxide and its monomeric forms, in an article published as early as 1938 in J.A.C.S., 1938, 61, p.1215-1225. More recently, Hsu et al. showed that stable polymerized solutions of ferric salts could be obtained by heating dilute solutions of FeClO.sub.4 to different temperatures, the nature and stability of the hydrolyzates depending on the temperature, and the concentration of iron. J. Soil Sci., 1972, 23, No. 4, p. 49; Clay & Clay Minerals, 1973, 21, p. 367-377; Soil Sci. Soc. of Am. H, 1980, 44, No. 1, p. 143-149, "Effect of Temperature on the Appearance and Stability of Hydrolyzed Fe (ClO.sub.4).sub.3 Solutions".
According to these various publications, the key factor governing the appearance and the stability of the solutions seems to be the initial degree of oversaturation of the solution in relation to the amorphous hydroxide.
Besides, it is possible to obtain these various compounds by neutralizing the ferric chloride with soda or lime. Their use as replacement for ferric chloride alone as coagulants has already been considered by many authors. P. Rengasamy et al., Aus. J. Soil Res., 1977, 15, p. 1-13; W. Stumm et al., J.A.W.W.A., 1968, 60, p. 514, "Stoichiometry of Coagulation"; W. Stumm et al., J.A.W.W.A., 1962, 54, p. 971-994, "Chemical Aspects of Coagulation".
The use of these solutions has proved to be of little interest for cost reasons, while the results obtained were not significantly different from those obtained by using only ferric chloride.